(a) Field of the Invention
The present invention relates to a photographic lens system for compact cameras with a field angle 70.degree. or more and aperture ratio F/3.5.
(b) Description of the Prior Art
Tessar type lens systems and various other types of lens systems are known as photographic lens systems for compact cameras. However, the field angle of known photographic lens systems is about 60.degree. to 65.degree. at the largest.
Generally, for photographic lens systems, astigmatism and distortion come into problem when it is attempted to make the field angle large. Therefore, even when curvature of field is corrected by making Petzval's sum small, astigmatic difference increases. For photographic lens systems for compact cameras, a very high price is not desirable. Therefore, it is not preferable to correct astigmatic difference by adopting a complicated lens configuration. Even when it is permitted to make the lens configuration complicated, lenses should be arranged in a very limited space because the overall length of the lens system cannot be made long in case of lens systems for compact cameras and, cosequently, the lens configuration cannot be made complicated in practice. Therefore, for photographic lens systems for compact cameras, it is difficult to correct astigmatic difference.
In recent years, photographic lens systems of so-called telephoto type with comparatively simple lens configuration is known, i.e., such lens systems are arranged that the front lens group has positive power and rear lens group has negative power in order to make the overall length of the lens system short. For example, the lens system disclosed in Japanese published examined patent application No. 50248/81 is known as a lens system arranged as descrived in the above.
However, in case of the above-mentioned lens system, the front lens group has strong positive power and, therefore, positive distortion is caused by the front lens group. Besides, as a negative meniscus lens which is convex toward the image surface is arranged on the image side of the stop, opsitive distortion occurs largely at said negative meniscus lens when the field angle becomes larger and the incident angle of principal ray on said negative meniscus lens becomes larger. It is difficult to correct the above-mentioned positive distortion.
Besides, the lens system disclosed in Japanese published unexamined patent application No. 95207/81 is known as a wide-angle photographic lens system of telephoto type wherein the rear lens group consists of two lens components, i.e., a positive lens component and a negative lens component. However, the field angle of said lens system is about 60.degree..
When a compact camera is arranged that the stop serves also as the shutter, it is practically difficult to arrange the mechanism so as to move the stop. Therefore, lens systems for compact cameras are arranged that the stop is fixed and lens components are partially moved for the purpose of focusing. Especially, the focusing method to move the front lens or front lens group is adopted in most cases. As other known focusing methods, a method to move the intermediate lens is disclosed in Japanese published unexamined patent application No. 76148/79 and a method to move the rear lens is discolsed in Japanese published unexamined patent application No. 133711/81. However, in case of these focusing methods, it is impossible to perfectly correct variation of aberrations.